1. Technical Field
The present invention relates to a delivery system for transporting a medical interventional device to a target site within the body of a patient. More particularly, the invention relates to a system of coaxial tubular members for delivering a medical device, such as a stent, into a designated lumen in the body of a patient to establish, or maintain, patency of the lumen.
2. Background Information
In modern medicine, interventional devices are often percutaneously introduced into the body of a patient via a suitable delivery apparatus, and delivered to a target site within the body for a medical purpose. One common example of a medical interventional device introduced in this manner is a stent. A stent is typically inserted into the lumen of a vessel or other bodily passageway to reinforce, repair, or otherwise provide support to establish or maintain the patency of the lumen. For example, when a patient suffers from atherosclerosis (hardening of the arteries), a stent may be placed in a coronary or a peripheral artery at a location where the artery is weakened, damaged or otherwise susceptible to collapse. The stent, once in place, reinforces that portion of the artery, thereby restoring normal blood flow through the vessel.
One form of stent which is particularly desirable for implantation in arteries and other body lumens is a cylindrical stent which is radially expandable upon implantation from a smaller first diameter to a larger second diameter. Radially expandable stents are typically loaded onto, or into, a delivery catheter, and fed internally through the arterial pathways of the patient until the unexpanded stent reaches the target site. Radially expandable stents are normally of two general types. One type, generally referred to as a “balloon-expandable” stent, is fitted in a compressed state over an uninflated balloon at the distal end portion of the delivery catheter. Once the catheter reaches the target site, the balloon is inflated by transmitting an inflation fluid through a lumen in the delivery catheter to the interior of the balloon. Upon inflation, the balloon exerts a radial pressure on the stent, thereby causing the compressed stent to radially expand to a larger diameter. Following expansion, the stent exhibits sufficient radial rigidity to remain in the expanded condition after the balloon has been deflated and the catheter has been removed.
The other type of radially expandable stent, generally referred to as a “self-expanding” stent, is formed from a resilient or shape memory material which is capable of self-expanding from a compressed state to an expanded state without the application of a radial outwardly-exerted force on the stent. Typically, a self-expanding stent is loaded into a delivery device that restrains the stent in the compressed state. Once the delivery device is directed to the target site, an ejection mechanism, such as a pusher, is employed to eject the stent from the distal end of the delivery device. Alternatively, an outer sheath of the delivery device is withdrawn such that it no longer covers the stent. In either event, once the stent is freed from the restraints of the device, it self-expands to the desired diameter.
The use of radially expandable stents advantageously allows the physician to insert relatively smaller diameter medical devices to prop up, reinforce or otherwise support relatively larger diameter vessels. However, the delivery of such stents to the target site has at times proven to be problematic. For example, the structure of a conventional delivery catheter may cause the catheter shaft to be subject to stress risers as it traverses the vessel. Stress risers comprise weakened or high stress segments of the catheter which may cause the catheter shaft to undesirably bend or otherwise fail during passage through the vessel. In a delivery system for a self-expanding stent that includes coaxial catheters, such high stress segments may occur, for example, at the point where the inner catheter meets the outer catheter. This typically occurs when the delivery system traverses a tortuous pathway in the body of the patient. In some cases, such as the bifurcation into the iliacs, the catheter is required to go around a high angle bend. In these instances, kinking and decreased trackability are prone to occur due to the high stress in the system as it attempts to traverse the high angle bend.
It is desired to provide a delivery system for a stent or other interventional medical device that avoids the problems of prior art devices.